Problem with Faxing using Windows Fax Software

We developed a function based on information in this forum that allows sending automatic mass faxes to customers. It almost works, even on RTC where the .pdf file is being generated on the service thier and then downloaded to the client.

But when we try to submit the fax to the fax software we get the error message "ConnectedSubmit failed: Operation failed". It works when I use a fax that is installed on my client and that has been mapped to my RDP session. But as soon I try to use the fax on the client I get this message.

I assume it is either a driver or permission issue, but I can manually send a fax so it does not make sense. Any ideas?